Add solutions for common errors

poetry/console/config/application_config.py

@@ -29,6 +29,7 @@
 from poetry.console.commands.env_command import EnvCommand
 from poetry.console.logging.io_formatter import IOFormatter
 from poetry.console.logging.io_handler import IOHandler
+from poetry.utils._compat import PY36
 
 
 class ApplicationConfig(BaseApplicationConfig):
@@ -46,6 +47,15 @@ def configure(self):
         self.add_event_listener(PRE_HANDLE, self.register_command_loggers)
         self.add_event_listener(PRE_HANDLE, self.set_env)
 
+        if PY36:
+            from poetry.solution_providers import (
+                PythonRequirementIncompatibilitySolutionProvider,
+            )
+
+            self._solution_provider_repository.register_solution_providers(
+                [PythonRequirementIncompatibilitySolutionProvider]
+            )
+
     def register_command_loggers(
         self, event, event_name, _
     ):  # type: (PreHandleEvent, str, Any) -> None

poetry/mixology/failure.py

@@ -2,6 +2,8 @@
 from typing import List
 from typing import Tuple
 
+from poetry.core.semver import parse_constraint
+
 from .incompatibility import Incompatibility
 from .incompatibility_cause import ConflictCause
 from .incompatibility_cause import PythonCause
@@ -44,10 +46,15 @@ def write(self):
                     )
                     required_python_version_notification = True
 
+                root_constraint = parse_constraint(
+                    incompatibility.cause.root_python_version
+                )
+                constraint = parse_constraint(incompatibility.cause.python_version)
                 buffer.append(
-                    "  - {} requires Python {}".format(
+                    "  - {} requires Python {}, so it will not be satisfied for Python {}".format(
                         incompatibility.terms[0].dependency.name,
                         incompatibility.cause.python_version,
+                        root_constraint.difference(constraint),
                     )
                 )
 

poetry/mixology/version_solver.py

@@ -11,7 +11,6 @@
 from poetry.core.packages import ProjectPackage
 from poetry.core.semver import Version
 from poetry.core.semver import VersionRange
-from poetry.puzzle.provider import Provider
 
 from .failure import SolveFailure
 from .incompatibility import Incompatibility
@@ -40,7 +39,7 @@ class VersionSolver:
     def __init__(
         self,
         root,  # type: ProjectPackage
-        provider,  # type: Provider
+        provider,
         locked=None,  # type: Dict[str, Package]
         use_latest=None,  # type: List[str]
     ):

poetry/solution_providers/__init__.py

@@ -0,0 +1,3 @@
+from .python_requirement_incompatibility_solution_provider import (
+    PythonRequirementIncompatibilitySolutionProvider,
+)

poetry/solution_providers/python_requirement_incompatibility_solution_provider.py

@@ -0,0 +1,32 @@
+import re
+
+from typing import List
+
+from crashtest.contracts.has_solutions_for_exception import HasSolutionsForException
+from crashtest.contracts.solution import Solution
+
+
+class PythonRequirementIncompatibilitySolutionProvider(HasSolutionsForException):
+    def can_solve(self, exception):  # type: (Exception) -> bool
+        from poetry.puzzle.exceptions import SolverProblemError
+
+        if not isinstance(exception, SolverProblemError):
+            return False
+
+        m = re.match(
+            "^The current project's Python requirement (.+) is not compatible "
+            "with some of the required packages Python requirement",
+            str(exception),
+        )
+
+        if not m:
+            return False
+
+        return True
+
+    def get_solutions(self, exception):  # type: (Exception) -> List[Solution]
+        from poetry.solutions.python_requirement_compatibility_solution import (
+            PythonRequirementCompatibilitySolution,
+        )
+
+        return [PythonRequirementCompatibilitySolution(exception)]